Defrost the peas. Melt the fat and sweat the flour in it. Deglaze with milk and stock while stirring, bring to the boil. Cut 4 cheese slices smaller and melt in the sauce. Season to taste with salt and pepper. Whisk the egg yolks and stir into the sauce. Do not let it boil any more!
Cut the croissants, but do not cut through. Cut remaining cheese slices diagonally in half. Cover the croissants with 2 cheese wedges and 1 slice of salmon. Sprinkle with peas. Spread the sauce in casseroles (1 large or 2 smaller ones) and place the croissants inside. Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for 8-10 minutes. Garnish with dill